The present invention relates to a method of optical lithography and a projection printing apparatus.
The present invention is effective for forming a pattern with high resolution, and more particularly, is best suited for a printing technique in a manufacturing process of semiconductor integrated circuits of large scale integration (LSI) and very large scale integration (VLSI), etc.
Optical lithography has been widely used as a lithography technology for manufacturing semiconductor integrated circuits, for example, as disclosed in the "Nikkei Microdevice for February, 1987" pp. 103-124 and the "Hitachi Technical Review Vol. 71, No. 5 (1989-5)" pp. 25-32.
FIG. 2 shows an outline of a projection printing apparatus which is used for printing a circuit pattern onto a photosensitive layer on a wafer.
In FIG. 2, emitted light from a light source 1 is converged by a reflecting mirror 2, and then, applied to a fly's eye lens or lens array 3. An image 8 of a secondary light source is formed on a focal surface of the fly's eye lens 3. The secondary light source image is irradiated to a mask or reticle 5 on which a desired pattern consisting of a light transmitting portion and a light non-transmitting portion is formed by means of a condenser lens 4, and the image is formed on a pupil 9 of a projection optical system 6.
The projection optical system 6 projects the image of the reticle 5 onto a wafer 7, thereby to form a latent image corresponding to the mask image on a photosensitive layer on the wafer 7. In this case, the image formed by the condenser lens 4 of the secondary light source image 8 is formed to the full extent of the pupil 9 of the projection optical system, so that the image forming capability of the projection optical system 6 can be used sufficiently.
In the printing technique described above, a resolution of a reticle image projected on the wafer is determined by a numerical aperture NA of the projection optical system and a wavelength .lambda. of a light source, and the resolution being almost in proportion to .lambda./NA when an illumination system such as the condenser lens 4 and the projection optical system 6 are aberration free.
The configuration of the pattern formed on a photosensitive layer on the wafer is also affected by a development process other than the printing process. In the case that the effect is expressed by a process parameter k, the resolution of the pattern becomes k.lambda./NA.
Hereupon, the value of k has been obtained approximately 0.7 at present, but, it is required that further improvement of both the resolving power of the projection optical system 6 and the process parameter k is necessary in the case that the pattern density is enhanced.